Private line multipoint data communications circuits are used in real-time systems in which a central processing terminal (a computer, for example) communicates with a plurality of remote terminals over a shared two-way communications facility. Typical multipoint systems include news wire services, airline reservation services and on-line banking services.
Multipoint circuits often involve a polling protocol where the central processing unit (CPU) transmits data over the shared facility addressed to an single remote terminal with only the remote terminal addressed returning data to the CPU. At any particular time, the equivalent of a point-to-point circuit connection exists between the CPU and a single remote terminal.
A multipoint data bridge interconnects the backbone transmission facility and one or more of the remote terminals on a branch of the transmission facility. It is generally located in a telephone central office or at a special private line service center. Each branching location of the backbone facility includes a bridge assembly which further comprises two electrically independent bridges. These bridges are identified as the distribution bridge and the collection bridge. The distribution bridge connects and outgoing or downstream transmission path to the receiver side of remote terminals. The collection bridge connects the transmitter side of remote terminals to the incoming or upstream transmission path. At the collection bridge, to which this invention relates, one or more remote terminal branches are connected through equalizing, balancing and impedance matching pads to the upstream transmission path. Under ideal conditions only response signals of short duration from the addressed remote terminals should appear at any collection bridge. However, under practical conditions two significant troubles can manifest themselves. There exist the noise-buildup and streaming troubles that are difficult to assign to a particular remote terminal branch without individual diagnoses of the respective remote terminal branches.
Noise buildup results from the summation at a collection bridge of all the noise energy arising in the branches connected directly to that collection bridge or indirectly through other intermediate collection bridges. Individual noise sources are difficult to trace, but the origin of such noise is usually ascribed to poor cable connections.
Streaming is a trouble condition in which a remote modem (data set) is transmitting an unmodulated carrier signal continuously because of a control failure at the terminal, or a failure in the modem connected to it.
Various proposals have been made to overcome the noise-buildup and streaming troubles. One approach would only connect to the collection bridge that branch which has a predetermined energy level present and lock out all other branches. Another proposal is that of addressable switching whereby only a remote terminal addressed through the distribution bridge by the CPU would be connected to the collection bridge. This approach is code sensitive and therefore in conflict with the goal of keeping the bridges as transparent as possible at all times to signal energy.
It is accordingly an object of this invention to improve the performance of and simplify maintenance problems for private line multipoint voiceband data services.
It is another object to provide for automatic fault isolation on analog multipoint data circuits.
It is a further object to reduce downtime for trouble diagnosis on analog multipoint data circuits.